During oil and gas exploration, many types of information are collected and analyzed. The information is used to determine the quantity and quality of hydrocarbons in a reservoir, and to develop or modify strategies for hydrocarbon production. One technique for collecting relevant information involves logging or monitoring the electromagnetic (EM) fields in a formation. Such systems must overcome a number of obstacles, including high temperatures, vibrations, space limitations, etc., and development efforts to address these concerns are ongoing. One vexing issue for many such systems is the requirement for a downhole EM source, which necessitates a sizeable downhole energy source with a limited lifetime or with some cumbersome method for supplying power from the surface. The suggested use of an ambient EM signal energy, such as the Earth's magnetotelluric currents, faces certain feasibility issues arising from low signal strength and poor source localization.
It should be understood, however, that the specific embodiments given in the drawings and detailed description below do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and other modifications that are encompassed in the scope of the appended claims.